Method and apparatus for burying pipeline

ABSTRACT

A pipeline is buried in the bottom of a body of water by fluidizing sediments which compose the bottom adjacent the pipeline so that the pipeline sinks by its own weight into the fluidized sediments.

I United States Patent 1 1 3,659,425 De Geeter [451 May 2, 1972 54]METHOD AND APPARATUS FOR [56] References Cited BURYING PIPELINE UNITEDSTATES PATENTS [72] Inventor: Pieter J. De Geeter, Rijswijk, Netherlands3,505,826 4/1970 Harmstorf ..6l/72.4 1 Asslgnw Shell Company, New York,3,217,499 11/1965 lshiki ..6l/72.4 [22] Filed: May 25, 1970 3,256,6956/1966 Bodine, Jr ..6l/72.4 X

[211 Ap No.: 40,407 Primary Examiner-Jacob Shapiro Att0rneyJ. H.McCarthy and Theodore E. Bieber [30] Foreign Application Priority DataMay 23, 1969 Great Britain ..26,392/69 [57] ABSTRACT A pipeline isburied in the bottom of a body of water by [52] US. Cl ..6l/72.4, 37/61fluidizing sediments which compose the bottom adjacent the lllt- 1302f5/ pipeline so that the pipeline sinks by its own weight into the [58]Field of Search ..6 1/724, 72, 53.74; 173/49; fl idized di 3 Claims, 3Drawing Figures PATENTEDMAY 2 I972 INVENTOR:

P. J- DE GEETER METHOD AND APPARATUS FOR BURYING PIPELINE BACKGROUND OFTHE INVENTION 1 Field of the Invention The invention relates to a methodof burying a pipeline in the bottom of a body of water, for example, inthe seabed.

2. Description of the Prior Art The conventional manner of burying apipeline in the bottom of a body of water comprises making a trench,laying the pipeline in the trench and allowing the trench to fill upagain. This is a rather complicated and expensive method, which has thedisadvantage that often the trench fills up again before the pipelinecan be sunk into the trench.

SUMMARY OF THE INVENTION It is an object of the invention to bury apipeline in the bottom of a body of water, without it being necessary tomake a trench, so that the disadvantages thereof are avoided.

For this purpose, the method according to the invention comprisesfluidizing by fluid injection the bottom of the body of water adjacentthe pipeline by moving a fluidization device along the pipeline, thefluidization device extending over such a minimum length of the pipelinethat the latter is buried to the desired depth in one pass, whereinfluid is supplied to the fluidization device, said fluid being passed bythe fluidization device to the bottom adjacent the pipeline in such away as to cause fluidization of the bottom adjacent the pipeline atleast over the said length, wherein said length is governed by theformula:

E modulus of elasticity of the pipe material,

I=lii1ear moment of inertia of the pipe,

q net weight of the pipe (with fluidization device) relative to thefluidized bottom per unit of length of pipe,

I= length of the fluidized bottom adjacent the pipeline,

Z depth of burying the pipeline,

C a constant having a value of about 60.

An apparatus for carrying out the method according to the inventioncomprises a fluidization device adapted to be arranged on a pipeline,means for moving the fluidization device along the pipeline, and meansfor supplying fluid to the fluidization device. The fluidization deviceis provided with nozzles for directing fluid jets into the bottomadjacent the pipeline in such a way as to cause fluidization of thebottom adjacent the pipeline. The length of the fluidization device isgoverned by the formula:

4 l Z wherein BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematicside view partially in outline form of an apparatus for carrying out themethod of the invention.

FIG. 2 is a side view of a fluidization unit of the apparatus accordingto FIG. 1.

FIG. 3 is a cross section taken along the line 3-3 of the fluidizationunit according to FIG. 2.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIG. 1, we see thefloor or bottom I of a body of water 5. The bottom 1 may consists of anunconsolidated material such as sand or clay or other sediments. The topsurface of the bottom 1 is indicated by the reference number 2 and thesurface of the water by the number 3.

On a pipeline 13, supported by the bottom 1, are arranged a number offluidization units 4 (some of which are shown in outline only) which areof a construction as shown in FIGS. 2 and 3. The fluidization unit 4 issubstantially O-shaped (see FIG. 3) in cross section. It consists of anumber of longitudinal parallel metal tubes 15, 16, 17, interconnectedby metal tubes such as tubes 18, 18a, 19, and 19a. Between tubes 16 and17 are arranged rollers 20, between tubes 15 and 16 are arranged rollers21 and between tubes are arranged rollers 22. Furthermore, one or moreextra rollers may be present.

The lowest longitudinal tubes 17 are provided with a large number ofnozzles 23 and 24 (see FIG. 3), directed inwardly and/or downwardly.Upwardly directed nozzles may be present as well. The longitudinal tubes15 are provided with a fluid inlet 25. All the metal tubes l5, l6, l7,18, 18a, 19, and 19a are interconnected in such a way that fluid canflow from the inlet 25 and through said tubes to the nozzles 23 and 24.

Normally, the metal tubes 18 and 19 are provided as well with nozzles(not shown), directed for fluidization of soil in front of the units toobtain horizontal movement of the units at minimum pulling resistance.

From FIGS. 2 and 3, it will be clear that the fluidization unit 4 can belowered on the pipeline 13 so that it will surround the pipeline 13, inthe way as most clearly shown in FIG. 3. The rollers 20, 21 and 22pennit displacement of the fluidization units 4 along the pipeline 13 asrequired.

Reverting to FIG. I, a large number l l of the fluidization units 4 areput on the pipeline 13. The units 4 are interconnected in a suitablemanner with a connecting means. for example, by one long chain or cable11, or by a number of chains or cables 11 between each pair of units 4.To the foremost unit 4 is secured a cable or chain 14 which is connectedto a tugboat (now shown), or any other pulling device such as forexample a winch on shore, or a winch on the bottom.

A barge 7 provided with a number of pumps 8 is floating on the watersurface 3. The pump suction lines are indicated by the reference number12 and the pump pressure lines are indicated by the reference number 9.The pressure lines 9 are connected via valves 10 to conduits 6, whichare preferably in the shape of hoses. The other ends of the conduits orhoses 6 are each connected to a fluid inlet 25 of a correspondingfluidization unit 4.

The apparatus, as described, is used in the following manner. Assumethat a pipeline I3 is laying on the top surface 2 of the bottom 1. Thena tugboat (not shown) draws, via the cable 14, the fluidization units 4along the pipeline 13. During this displacement fluid, preferably water,is continuously supplied by the pumps 8, through the hoses 6, to thefluidization units 4. This causes water to flow continuously from thenozzles 23 and 24. This water ejecting from these nozzles 23 and 24causes the soil of the bottom to be fluidized. This means that thebottom material adjacent to (that is under and along the sides of) thepipeline 13 assumes the properties of a liquid, so that the pipeline 13will sink under the influence of its own weight into said bottommaterial until it has reached the desired depth.

By means of the method according to the invention, it is possible tocause the pipeline 13 to sink to the desired depth in the bottom 1 bymoving the fluidization units 4 along the pipeline 13 once. In otherwords, the pipeline 13 can be buried in one pass, at the desired depthin bottom 1.

In order to obtain the required burying depth of the pipeline in onepass and not to overstrain the pipeline 13, it is, however, necessary tofluidize the bottom material over a sufficient length of the pipeline.This means that a fluidization device of sufficient length or asufficiently large number of fluidization units 4 has to be used, inorder to be able to cover said length. The minimum required length ofthe fluidization device can be calculated from the formula:

E modulus of elasticity of the pipe material,

I linear moment of inertia of the pipe,

q net weight of the pipe (with fluidization device) relative to thefluidized bottom per unit of length of pipe,

I= length of the fluidized bottom under the pipeline,

2 depth of burying the pipeline,

C= a constant having a value of about 60.

Each fluidization unit 4 has its own supply of fluid in order to be ableto control the degree of fluidization per unit. In FIG. 1 each unit 4 isshown as having its own conduit 6. Of course, it is possible to have,instead, only one conduit supplying fluid to all the fluidization units4. In that case, a suitable system of valves (not shown) will be neededto maintain the possibility of independent control of each unit 4.

By way of illustration, the following example is given of a steelpipeline of 36-inch diameter with concrete coating having a linearmoment of inertia of I 400,000 cm and being made of such a material thatthe modulus of elasticity is E 2.l X N/cm.

It is desired to bury this pipeline to a depth Z 300 cm in a seabed.

q volume xp weight of fluidization units/m q 660 dm (p pipe pbed) weightof fluidization units/m 660 (1.4 1.4) 16,000 N/8m 2,000 N/m 20 N/cm (Nis about 0.1 kgf) By substituting the above data in the formula whereinC has the value of 60, it is found that for this particular case l= 93meters.

in FIG. 1 a barge 7 is shown carrying pumps 8. Instead it is possible tomount on each unit 4 a pump (not shown). Such a pump could suck in waterfrom around the pipeline l3 and press it via inlet 25 to the nozzles 23and 24. In such an embodiment, the pumps 8 and conduits 6 could beomitted.

Furthermore, in FIG. 1, the train of fluidization units 4 is drawn alongthe pipeline 13 by means of, a tugboat and the cable 14. Of course, itwould be possible to use a barge with winches instead of a tugboat.Instead it would be possible to put a kind of a locomotive (not shown)on the pipeline 13 adapted to draw the train of units 4 along thepipeline 13. Such a locomotive could, for example, be provided with asuitable electric motor and could be controlled through an electricpower cable from a ship. The fluid to be used is preferably water.Instead it is, however, possible to use a mixture of air and water, ifdesired.

If desired, sonar equipment can be used to determine, during the buryingoperation, the level of each fluidization unit relative to the pipelinealso in case a fluidization unit is fully sunk into the bottom. Use ofsuch sonar equipment makes it also possible to'control the degree offluidization under each fluidization unit in dependence of the dataproduced by the sonar equipment. In other words, if the sonar equipmentindicates that one or more of the fluidization units are not suffcientlydeep below the top surface of the bottom then extra fluid can besupplied to the said fluidization units until the I desired depth isreached.

The fluidization device may consist of a single unit of a minimumlength, governed by the formula as mentioned above. Instead a train ofinterconnected fluidization units as described may be used having atotal length as governed by the formula as mentioned above.

The fluidization device should be of such a weight that its weight perunit of length is sufficient to render the local specific gravity of thepipeline sufficient together with that of the fluidization device toguarantee descent of the pipeline in the fluidized bottom material. I

In the embodiment of the method of the invention as described, thepipeline is first laid on the bottom of the body of water, whereafterthe fluidization device is moved along the pipeline to bury it. Instead,it is possible to move the fluidization device along the pipeline duringthe laying operation of the pipeline, so that the pipeline isimmediately buried into the bottom during the laying operation.

In order to prevent tilting of the fluidization units, each fluidizationunit should be dimensioned in such a way that, during normal use (thatis in the position as shown in FIG. 3), the center of gravity of theunit is laying below the center of the pipeline.

In summary, this invention provides a method and apparatus for burying apipeline in an unconsolidated material which forms a floor underlying abody of 'water. The method comprises the steps of: laying at least aportion of the pipeline upon the floor underlying said body of water,providing adjacent at least one selected interval of the portion of thepipeline lying upon the floor an apparatus capable of fluidizing atleast some of the unconsolidated material by jetting a fluid into theunconsolidated material, and jetting a fluid into the unconsolidatedmaterial adjacent the selected interval of the portion of the pipelinelying upon the floor whereby the unconsolidated material is fluidizedadjacent the selected interval of the pipeline sinks into the fluidizedunconsolidated material. In a preferred embodiment of the method, theapparatus may be moved along the pipeline while jetting the fluid.

One embodiment of an apparatus which may be used in the practice of thisinvention comprises a fluidization devide adapted to be axially movablyarranged on a pipeline, nozzle means operatively connected tofluidization device for direct ing a jet of a fluid into unconsolidatedmaterial adjacent the pipeline, means such as hoses or other conduits,for supplying fluid to the nozzles, and means for moving thefluidization device along the pipeline. The fluidization device maycomprise a plurality of hollow steel tubes joined together so as to forman elongate, open-framework structure'which in cross section describes ageometric figure having a partially open periphery whereby the structuremay be fitted over a pipeline.

I claim as my invention:

1. In a method for burying a pipeline a selected depth in anunconsolidated material which forms a floor underlying a' body of waterof the type comprising the steps of laying at least a portion of saidpipeline upon said floor underlying said body of water; providingadjacent at least one selected interval of said portion of said pipelinelying upon said floor an apparatus capable of fluidizing at least someofsaid unconsolidated material by jetting a fluid into said unconsolidatedmaterial; and jetting a fluid into said unconsolidated material adjacentsaid selected interval of said portion of said pipeline lying upon saidfloor whereby said unconsolidated material is fluidized adjacent saidselected interval of said pipeline and whereby at least some of saidpipeline sinks into said fluidized unconsolidated material; theimprovement which comprises the steps of:

determining a length of said selected interval of said pipeline adjacentwhich fluid is injected that is sufficient to allow said pipe to sink tosaid selected depth without overstraining said pipe; and

jetting said fluid adjacent a selected interval of said pipeline havinga length at least great as said above determined length.

2. The method of claim 1 including the step of moving said apparatusalong said pipeline while jetting said fluid to fluidize saidunconsolidated material.

3. The method of claim 1 wherein said determined length of said selectedinterval of said pipeline adjacent which fluid is injected is at leastas great as a length, I given by the formula:

E modulus of elasticity of the pipeline material,

I linear moment of inertia of the pipeline,

q =net weight of the pipeline relative to the weight of theunconsolidated material fluidized per unit of length of pipeline, 5

Z depth of burying the pipeline,

C= a constant having a value of about 60.

1. In a method for burying a pipeline a selected depth in anunconsolidated material which forms a floor underlying a body of waterof the type comprising the steps of laying at least a portion of saidpipeline upon said floor underlying said body of water; providingadjacent at least one selected interval of said portion of said pipelinelying upon said floor an apparatus capable of fluidizing at least someof said unconsolidated material by jetting a fluid into saidunconsolidated material; and jetting a fluid into said unconsolidatedmaterial adjacent said selected interval of said portion of saidpipeline lying upon said floor whereby said unconsolidated material isfluidized adjacent said selected interval of said pipeline and wherebyat least some of said pipeline sinks into said fluidized unconsolidatedmaterial; the improvement which comprises the steps of: determining alength of said selected interval of said pipeline adjacent which fluidis injected that is sufficient to allow said pipe to sink to saidselected depth without oversTraining said pipe; and jetting said fluidadjacent a selected interval of said pipeline having a length at leastgreat as said above determined length.
 2. The method of claim 1including the step of moving said apparatus along said pipeline whilejetting said fluid to fluidize said unconsolidated material.
 3. Themethod of claim 1 wherein said determined length of said selectedinterval of said pipeline adjacent which fluid is injected is at leastas great as a length, l , given by the formula: E modulus of elasticityof the pipeline material, I linear moment of inertia of the pipeline, qnet weight of the pipeline relative to the weight of the unconsolidatedmaterial fluidized per unit of length of pipeline, Z depth of buryingthe pipeline, C a constant having a value of about 60.